The present invention relates to global navigation satellite systems (GNSS), and more particularly, to methods and apparatuses for searching satellite signals.
Global navigation satellite systems (GNSS), such as Global Position System (GPS), Galileo, or GLONASS, are widely used in many applications. A GNSS receiver can determine its position by receiving and analyzing coded signals transmitted from a plurality of orbiting satellites. The amount of time it takes for the GNSS receiver to search for satellite signals and determine the initial position is called the time to first fix (TTFF). The TTFF is an important criterion for evaluating the performance of a GNSS receiver.
In order to reduce the TTFF, the GNSS receiver usually stores some navigation information (e.g. receiver position, time, ephemeris, almanac, receiver clock drift, receiver velocity, etc.) in a non-volatile memory unit. When activated, the GNSS receiver computes predictions of satellite measurements according to the navigation information stored in the memory unit. The predictions typically include satellites in-view, Doppler, and code chip phase. Ideally, these predictions can assist the GNSS receiver in searching for satellite signals to reduce the TTFF.
Unfortunately, the GNSS receiver may fail to achieve TTFF if it is under unfriendly RF conditions. As is well known, the GNSS receiver may be impossible to achieve a position fix in a building, a basement, and urban canyons, which are really RF-shadowed environments. In addition, if the navigation information stored in the memory unit is not consistent with the real operating conditions of the GNSS receiver, the GNSS receiver may also fail to achieve TTFF. For example, the previously obtained position stored in the memory unit is in the Southern Hemisphere while the GNSS receiver is actually in the Northern Hemisphere. Such inconsistency leads the GNSS receiver to generate incorrect measurement predictions, and the GNSS receiver may continue to search for satellites that lie below the horizon based on such incorrect measurement predictions. In these cases, the GNSS receiver cannot achieve a position fix.
If the GNSS receiver fails to achieve TTFF because the measurement predictions are incorrect, then the incorrect measurement predictions should be discarded. If, however, the GNSS receiver fails to achieve TTFF because the GNSS receiver is in an unfriendly RF environment then the measurement predictions should not be discarded. In the related art, however, it is difficult to identify the environment and why the GNSS receiver fails to achieve TTFF.